1. Field of the Invention
The present invention relates to a light-emitting semiconductor device using gallium nitride group compound which emits a blue light.
2. Description of the Prior Art
It is known that GaN compound semiconductor can be made into a light-emitting semiconductor device, such as a light-emitting diode (LED), which emits a blue light. The GaN compound semiconductor attracts attention because of its high light-emitting efficiency resulting from direct transition and of its ability to emit a blue light which is one of three primary colors.
The light-emitting diode manufactured from the GaN compound semiconductor is composed of an n-layer and an i-layer grown thereon. The n-layer of the GaN compound semiconductor with n-type conduction is directly grown on a surface of a sapphire substrate or grown on a buffer layer of aluminum nitride formed on the substrate. The i-layer of insulating (i-type) GaN compound semiconductor doped with p-type impurities is grown on the n-layer. (See Japanese Patent Laid-open Nos. 119196/1987 and 188977/1988.) The light-emitting diode of this structure has room for improvement in luminous intensity. In addition, it comprises no p-n junction but it is made by joining the i-layer and n-layer.
An electric property of the GaN compound semiconductor shows inherently n-type conduction even though it is not deliberately doped with n-type impurities, and unlike silicon and similar semiconductors, when it is doped with zinc of p-type impurities, the electric property shows not p-type conduction but insulation. Moreover, the production of n-type GaN involves many difficulties in controlling conductivity.
It is the first object of the present invention to improve a luminous efficiency of a GaN group light-emitting diode.
It Is the second object of the present invention to provide a new layer structure which improves a luminous efficiency of a GaN group light emitting diode.
It is the third object of the present invention to provide a technology for production of n-type GaN group compound semiconductor in which conductivity is easily controlled.
After experience in the manufacture of the above-mentioned GaN light-emitting diode, the present inventors established a technology for a vapor phase epitaxy of the GaN group semiconductor with organometal compound. This technology enables a production of a gas-phase grown GaN layer of high purity. In other words, this technology provides n-type GaN with high resistivity without doping with impurities, unlike the conventional technology which provides n-type GaN with low resistivity when no doping is performed.
The First Feature of the Invention
The first feature of the present invention resides in a light-emitting semiconductor device composed of an n-layer of n-type gallium nitride group compound semiconductor (AlxGa1xe2x88x92xN; inclusive of x=0) and an i-layer of insulating (i-type) gallium nitride compound semiconductor (AlXGa1xe2x88x92xN; inclusive of x=0) doped with p-type impurities, in which the n-layer is of double-layer structure including an n-layer of low carrier concentration and an n+-layer of high carrier concentration, the former being adjacent to the i-layer.
According to the present invention, the n-layer of low carrier concentration should preferably have a carrier concentration of 1xc3x971014/cm3 to 1xc3x971017/cm3 and have a thickness of 0.5 to 2 xcexcm. In case that the carrier concentration is higher than 1xc3x971017/cm3, the luminous intensity of the light-emitting diode decreases. In case that the carrier concentration is lower than 1xc3x971014/cm3, since the series resistance of the light-emitting diode increases, an amount of heat generated in the n-layer increases when a constant current is supplied to it. In case that the layer thickness is greater than 2 xcexcm, since the series resistance of the light-emitting diode increases, the amount of heat generated in the n-layer increases when the constant current is supplied to it. In case that the layer thickness is smaller than 0.5 xcexcm, the luminous intensity of the light-emitting diode decreases.
In addition, the n+-layer of high carrier concentration should preferably contain a carrier concentration of 1xc3x971017/cm3 to 1xc3x971019/cm3 and have a thickness of 2-10 xcexcm. In case that the carrier concentration is higher than 1xc3x971019/cm3, the n+-layer is poor in crystallinity. In case that the carrier concentration is lower than 1xc3x971017/cm3, since the series resistance of the light-emitting diode increases, an amount of heat generated in the n+-layer increases when a constant current is supplied to it. In case that the layer thickness is greater than 10 xcexcm, the substrate of the light-emitting diode warps. In case that the layer thickness is smaller than 2 xcexcm, since the series resistance of the light-emitting diode increases, the amount of heat generated in the n+-layer increases when the constant current is supplied to it.
In the first feature of the present invention, it is possible to increase an intensity of blue light emitted from the light-emitting diode by making the n-layer in double-layer structure including an n-layer of low carrier concentration and an n+-layer of high carrier concentration, the former being adjacent to the i-layer. In other words, the n-layer as a whole has a low electric resistance owing to the n+-layer of high carrier concentration, and hence the light-emitting diode has low series resistance and generates less heat when a constant current is supplied to it. The n-layer adjacent to the i-layer has a lower carrier concentration or higher purity so that it contains a smaller amount of impurity atoms which are deleterious to the emission of blue light from the light-emission region (i-layer and its vicinity). Due to the above-mentioned functions, the light-emitting diode of the present invention emits a blue light of higher intensity.
The Second Feature of the Invention
The second feature of the present invention resides in a light-emitting semiconductor device composed of an n-layer of n-type gallium nitride compound semiconductor (AlxGa1xe2x88x92xN; inclusive of x=0) and an i-layer of i-type gallium nitride compound semiconductor (AlxGa1xe2x88x92xN; inclusive of x=0) doped with p-type impurities, in which the i-layer is of double-layer structure including an iL-layer containing p-type impurities in comparatively low concentration and an iH-layer containing p-type impurities in comparatively high concentration, the former being adjacent to the n-layer.
According to the present invention, the iL-layer of low impurity concentration should preferably contain the impurities in concentration of 1xc3x971016/cm3 to 5xc3x971019/cm3 and have a thickness of 0.01 to 1 xcexcm. In case that impurity concentration is higher than 5xc3x971019/cm3, since the series resistance of the light-emitting diode increases, an initial voltage to start emitting light at increases. In case that the impurity concentration is lower than 1xc3x971016/cm3, the semiconductor of the iL-layer shows n-type conduction. In case that the layer thickness is greater than 1 xcexcm, since the series resistance of the light-emitting diode increases, the initial voltage to start emitting light at increases. In case that the layer thickness is smaller than 0.01 xcexcm, the light-emitting diode has the same structure as that of the conventional one.
In addition, the iH-layer of high impurity concentration should preferably contain the impurities in concentration of 1xc3x971019/cm3 to 5xc3x971020/cm3 and have a thickness of 0.02 to 0.3 xcexcm. In case that the impurity concentration is higher than 5xc3x971020/cm3, the semiconductor of the iH-layer is poor in crystallinity. In case that the impurity concentration is lower than 1xc3x971019/cm3, the luminous intensity of the light-emitting diode decreases. In case that the layer thickness is greater than 0.3 xcexcm, since the series resistance of the light-emitting diode increases, an initial voltage to start emitting light at increases. In case that the layer thickness is smaller than 0.02 xcexcm, the i-layer is subject to breakage.
In the second feature of the present invention, it is possible to increase an intensity of blue light emitted from the light-emitting diode by making the i-layer in double-layer structure including an iL-layer containing p-type impurities in comparatively low concentration and an iH-layer containing p-type impurities in comparatively high concentration, the former being adjacent to the n-layer. In other words, this structure (in which the i-layer adjacent to the n-layer is the iL-layer of low impurity concentration) enables electrons to be injected from the n-layer into the iH-layer of high impurity concentration without being trapped in the iL-layer and its vicinity. Therefore, this structure enables electrons to pass through the iL-layer of low impurity concentration, which is poor in luminous efficacy, adjacent to the n-layer, and to reach the iH-layer of high impurity concentration in which electrons emit light with a high efficiency.
The Third Feature of the Invention
The third feature of the present invention resides in a light-emitting semiconductor device composed of an n-layer of n-type gallium nitride compound semiconductor (AlxGa1xe2x88x92xN; inclusive of x=0) and an i-layer of i-type gallium nitride compound semiconductor (AlxGa1xe2x88x92xN; inclusive of x=0) doped with p-type impurities, in which the n-layer is of double-layer structure including an n-layer of low carrier concentration and an n+-layer of high carrier concentration, the former being adjacent to the i-layer, and the i-layer is of double-layer structure including an iL-layer containing p-type impurities in comparatively low concentration and an iH-layer containing p-type impurities in comparatively high concentration, the former being adjacent to the n-layer.
The third feature of the present invention is a combination of the first feature (the n-layer of double layer structure) and the second feature (the i-layer of double layer structure). Therefore, the n-layer of low carrier concentration, the n+-layer of high carrier concentration, the iL-layer of low impurity concentration, and the iH-layer of high impurity concentration should correspond to the respective layers as the first and second features. The carrier concentration and layer thickness are defined in the same manner as in the first and second features.
In the third feature of the present invention, it is possible to increase an intensity of blue light from the light-emitting diode by making the n-layer in double-layer structure including an n-layer of low carrier concentration and an n+-layer of high carrier concentration, the former being adjacent to the i-layer, and also by making the i-layer in double-layer structure including an iL-layer containing p-type impurities in comparatively low concentration and an iH-layer containing p-type impurities in comparatively high concentration, the former being adjacent to the n-layer.
In other words, the n-layer as a whole has a low electric resistance owing to the n+-layer of high carrier concentration, which makes it possible to apply an effective voltage to the junction between the iL-layer and n-layer of low carrier concentration. Having a low carrier concentration, the n-layer adjacent to the iL-layer does not permit non-light-emitting impurity atoms to diffuse into the iL-layer. In addition, this structure (in which the i-layer adjacent to the n-layer is the iL-layer of low impurity concentration) permits electrons to be injected from the n-layer into the iH-layer of high impurity concentration without being trapped in the iL-layer. Therefore, this structure permits electrons to pass through the iL-layer of low impurity concentration, which is poor in luminous efficacy, adjacent to the n-layer, and to reach the iH-layer of high impurity concentration in which electrons emit light with a high efficiency.
For this reason, the light-emitting diode of the present invention has a much higher luminous efficacy than the one having the conventional simple i-n junction.
The Fourth Feature of the Invention
The fourth feature of the present invention resides in a method of producing an n-type gallium nitride compound semiconductor (AlxGa1xe2x88x92xN; inclusive of x=0) from organometal compound by vapor phase epitaxy. This method comprises a step of feeding a silicon-containing gas and other raw material gases together at a proper mixing ratio so that the conductivity of the compound semiconductor is desirably controlled. The mixing ratio is adjusted such that silicon enters the layer of gallium nitride compound semiconductor grown by vapor phase epitaxy and functions as the donor therein. Thus it is possible to vary the conductivity of the n-type layer by adjusting the mixing ratio.
The Fifth Feature of the Invention
The fifth feature of the present invention resides in a method for producing a light-emitting semiconductor device. The method comprises two steps. The first step involves growing an n+-layer of high carrier concentration (which is an n-type gallium nitride compound semiconductor (AlxGa1xe2x88x92xN; inclusive of x=0) having a comparatively high conductivity) by vapor phase epitaxy from organometal compound. The vapor phase epitaxy is accomplished on a sapphire substrate having a buffer layer of aluminum nitride by feeding a silicon-containing gas and other raw material gases together at a proper mixing ratio. The second step involves growing an n-layer of low carrier concentration (which is an n-type gallium nitride compound semiconductor (AlxGa1xe2x88x92xN; inclusive of x=0) having a comparatively low conductivity) by vapor phase epitaxy from organometal compound. The vapor phase epitaxy is accomplished on the n+-layer formed by the first step by feeding raw material gases excluding the silicon-containing gas. The n-layer of double-layer structure can be produced by properly controlling the mixing ratio of a silicon-containing gas and other raw material gases.